When the output of your stock alternator can not keep up with the current draw of the accessories that you normally run. A good battery will help but it is only a buffer between the alternator and your accessories so if they are accessories that you run simultaneously for long periods of time, you need to consider upgrading.

For example, your '98 may have a 124 amp alt from the factory (not sure how common it is, but my '99 did). Thats actually a pretty decent sized alt. But a winch can draw 300+ amps under a full load. So the alt can't supply the full amperage the winch is drawing, which is why a good, possibly oversized or dual, battery setup helps. It supplies the extra power needed, then once the load goes away, it re-charges the battery.

So assuming you aren't getting obvious signs that your alt isn't cutting it (dimming lights, trouble starting if the jeep hasn't been running for a while, or something else), you need to figure out how much power the accessories you are adding are going to draw when running. Amps = Watts / Volts. For calculations I usually use 12 volts even though while running you will usually see 13-14. So for a pair of 55W lights,

55 W x2 = 110 Watts

110W / 12 = 9.17 Amps

Some accessories may actually have their amp draw listed and you can just use that. I think some compressors are marked this way.

So start totalling up your accessories and see how much load you are adding. There are estimates floating around for what the vehicle consumes on its own but I don't remember off the top of my head.

Also, depending on how much extra capacity you need there are fairly straight-forward 136 amp and 160 amp alternators that will bolt in with minor modifications, which would give you added output without breaking the bank.

Edit: Also, if you are upgrading the alternator, you should be upgrading the wiring as well!

Keep in mind that alternator output is rated at cruising speeds and idle output is significantly lower.
You could spend a lot of time working calculations but a good rule of thumb is that if your engine is idling and your voltage guage is dropping, you need to decrease the electrical load by turning off some stuff, or increase power production with a higher output alternator.

***Chris***

"You can set my jeep on fire and roll it down a hill,
But I still wouldn't trade it for a Coupe DeVille."

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When the output of your stock alternator can not keep up with the current draw of the accessories that you normally run. A good battery will help but it is only a buffer between the alternator and your accessories so if they are accessories that you run simultaneously for long periods of time, you need to consider upgrading.

For example, your '98 may have a 124 amp alt from the factory (not sure how common it is, but my '99 did). Thats actually a pretty decent sized alt. But a winch can draw 300+ amps under a full load. So the alt can't supply the full amperage the winch is drawing, which is why a good, possibly oversized or dual, battery setup helps. It supplies the extra power needed, then once the load goes away, it re-charges the battery.

So assuming you aren't getting obvious signs that your alt isn't cutting it (dimming lights, trouble starting if the jeep hasn't been running for a while, or something else), you need to figure out how much power the accessories you are adding are going to draw when running. Amps = Watts / Volts. For calculations I usually use 12 volts even though while running you will usually see 13-14. So for a pair of 55W lights,

55 W x2 = 110 Watts

110W / 12 = 9.17 Amps

Some accessories may actually have their amp draw listed and you can just use that. I think some compressors are marked this way.

So start totalling up your accessories and see how much load you are adding. There are estimates floating around for what the vehicle consumes on its own but I don't remember off the top of my head.

Also, depending on how much extra capacity you need there are fairly straight-forward 136 amp and 160 amp alternators that will bolt in with minor modifications, which would give you added output without breaking the bank.

Edit: Also, if you are upgrading the alternator, you should be upgrading the wiring as well!

Keep in mind that alternator output is rated at cruising speeds and idle output is significantly lower.
You could spend a lot of time working calculations but a good rule of thumb is that if you're engine is idling and your voltage guage is dropping, you need to decrease the electrical load by turning off some stuff, or increase power production with a higher output alternator.

Thanks for the great reply's. right now I'm not running much, but I was planing on adding some more lights soon. I guess Ill have to do the math. Ill have about 70 amsp in extra lights total and a winch ... guess its new alternator time.

I saw some people swap a wj or zj alts ... guess it time to do some searching and find the best bet, or go aftermarket.

I'm running cables from Kelley's WIP and couldn't be happier. great guy to talk to and he'll make sure you get set up properly. And unless you happen to have parts and tools laying around I don't think its possible to beat his prices (I tried, and couldn't).

I don't know if it will work for you since I believe the alternator output connection changed in '99, but I run a 160 amp alternator off a 5.9 Durango (01-03). There is also a ZJ alt that is 136 amp that I think has the old style connections. I can't remember if there is a 160 amp alt with the old style connections or not.

Additionally, if you are looking at upgrading the battery (or whenever that time rolls around) look into AGM batteries. The Diehard Platinum P4 gets great reviews around here, and is made by Odyssey. I run an O'reilly's branded AGM battery that is made by Deka, which is also supposed to be a pretty good battery. For offroad use, AGM batteries are much better for a number of reasons, so its also something to look at while you're doing the rest of the electrical system.

I'm running cables from Kelley's WIP and couldn't be happier. great guy to talk to and he'll make sure you get set up properly. And unless you happen to have parts and tools laying around I don't think its possible to beat his prices (I tried, and couldn't).

I don't know if it will work for you since I believe the alternator output connection changed in '99, but I run a 160 amp alternator off a 5.9 Durango (01-03). There is also a ZJ alt that is 136 amp that I think has the old style connections. I can't remember if there is a 160 amp alt with the old style connections or not.

Additionally, if you are looking at upgrading the battery (or whenever that time rolls around) look into AGM batteries. The Diehard Platinum P4 gets great reviews around here, and is made by Odyssey. I run an O'reilly's branded AGM battery that is made by Deka, which is also supposed to be a pretty good battery. For offroad use, AGM batteries are much better for a number of reasons, so its also something to look at while you're doing the rest of the electrical system.

Thanks so much for the info . I'm running a red top and I just changed the + & - wires and the ground to the block. I didn't know about the alt wire up grade .... I might do tat tomorrow .... I'm going to search that now